1. Field of the Invention
This invention relates to systems for attenuating undesired interfering signals and, more particularly, to apparatus for substantially attenuating an undesired signal interfering with a Loran C radionavigation system.
2. Description of the Prior Art
To determine the approximate location of a ship or other mobile vehicle a Loran C receiving apparatus may be employed in such vehicle. A typical Loran C radionavigation system includes a master transmitting station and at least two secondary transmitting stations situated at different preselected locations. Each station transmits a pulse group, that is, a series of eight radio frequency pulses, during different nonoverlapping time intervals. More specifically, the master transmitting station transmits a pulse group followed in time by a transmission of a pulse group by the first secondary station. After the first secondary station has transmitted a pulse group, the second secondary station transmits a pulse group. This sequence of pulse transmission by the master and secondary stations continues ad infinitum.
The time difference (TD) is defined to be the difference in time between the arrival of the master station signal and one of the secondary station signals at the Loran C receiver site. The time difference associated with the first secondary station and the time difference associated with the second secondary station each varies according to the particular location at which the Loran C receiver is situated. Each time difference is typically displayed by the Loran C receiver such that two intersecting hyperbolic lines of position (LOP) may be drawn on a map to determine the location of the Loran C receiver site. For a more complete discussion of the Loran C radionavigation scheme, refer to U.S. Patent, Jasper et al., Ser. No. 222,422, filed concurrently herewith and assigned to the instant assignee.
A Loran C receiver often must operate in a harsh receiving environment. That is, undesired extraneous radio frequency signals may be present near or in the passband of the Loran C receiver (typically 90 kHz-110 kHz). To determine the above-discussed time differences, a Loran C receiver typically locks to the third positive-going zero crossing of the carrier of each of the pulses of the pulse group transmitted by each of the master and secondary stations, respectively. Such zero crossing is known as a pulse tracking reference point or PTR. The presence of extraneous signals will interfere with and, in extreme cases, completely prevent locking to the pulse tracking reference points of the Loran C pulse groups resulting in decreased accuracy of location determination or complete prevention of location determination. It is clear that attenuation of such extraneous interfering signals is desirable.
In one conventional Loran C receiving apparatus, a tunable bandpass filter is employed in conjunction with a tunable notch filter to attenuate undesired radio frequency signals interfering with a Loran C signal. The tunable bandpass filter is connected to a meter scaling and drive circuit which is coupled to a meter within view of the receiver operator. The operator adjusts the tunable bandpass filter and reads the relative amplitude of the interfering signal on the meter. The tunable notch filter is then adjusted to remove the interference. Although such a notch filter tuning arrangement performs well to attenuate interfering signals occurring outside of the passband of the receiver (again typically 90-110 kHz), such a filter arrangement causes undesirable Loran C pulse distortion when the notch filter is employed to attenuate a signal within the Loran C passband.
Another receiving apparatus conventionally employed to receive Loran C signals includes an interference canceller which operates to attenuate undesired signals within the Loran C signal bandwidth. A voltage controlled oscillator is phase locked to the interfering signal. The amplitude of the oscillator output signal is adjusted such that when the oscillator output signal is combined with the Loran C signal and the interfering signal at a differential amplifier, the oscillator output signal and the interfering signal cancel such that the Loran C signal remains without interference. This cancellation technique may be employed to facilitate its implementation.
It is one object of the present invention to substantially attenuate undesired radio frequency signals appearing within the Loran C bandwidth in a manner that may be digitally implemented.
Another object of the invention is to substantially attenuate signals interfering with a desired Loran C signal without distorting the Loran C pulses.
These and other objects of the invention will become apparent to those skilled in the art upon consideration of the following description of the invention.